The old Boral quarry, located on private property at Mt Gravatt East, is being used by trespassers as a playground to swim, cliff dive - even parasail. Photo: Supplied

Brisbane Deputy Mayor Adrian Schrinner said people needed to realise it was unsafe to trespass or swim in the disused quarry.

"Boral was required to carry out water testing a number of years ago and the water was found to be unsuitable for human contact," he said.

"It is my understanding that the water has been affected by minerals leaching out of the rocks and that the water may cause skin irritation for some individuals. If the water is accidentally ingested, it may also make people sick."

A Brisbane City Council report showed the water had a pH level of about 3.3, which was highly acidic, and also contained traces of metals "in excess of acceptable levels" and low nutrient levels.

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Cr Schrinner said he believed the majority of trespassers knew they were not permitted to enter the quarry.

"Kids will be kids, however, the key issue here is one of safety," he said.